The book is famous as being the first scientific book which used photographs as a major part of the evidence. Emotions, he decided, were behavioural traits which evolved. Darwin pointed out how the human face is adapted to show many of these emotions: it has muscles for facial movements which are not possible in other mammals. On the other hand, other mammals do have ways of showing many of these emotions.

Ekman has taken this idea a step further by studying the way people try to hide their emotions. He filmed the tell-tale glimpses of brief moments when true emotion is shown on a person's face.[3]